The dehydration of some calcium aluminate hydrates, with
univalent anions
Citation for published version (APA):
Houtepen, C. J. M., & Stein, H. N. (1976). The dehydration of some calcium aluminate hydrates, with univalent anions. Journal of Colloid and Interface Science, 56(2), 370-376. https://doi.org/10.1016/0021-9797(76)90264-2
DOI:
10.1016/0021-9797(76)90264-2
Document status and date: Published: 01/01/1976
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Hydrates, with Univalent Anions
C. J. M. HOUTEPEN AND H. N. STEIN
Laboratory of General Chemistry, Eindhown University of Techrtology, Eindlzonen, The Netherlands Received August 31, 197.5; December 12, 1975
The enthalpy of removing interlayer water of the calcium aluminate hydrates of the type 2(CazAl(OH)6+X--yHaO) (y=2, if X-=W, Br-, J-, NOs-, Clog-, C104; 2<y<4 if X-=Br03-, JOs- at 37% r-lative humidity and room temperature) is related to the enthalpy of hydration of the anion X-.
The enthalpy of dehydration of the hydrates cannot be determined as isosteric heat of adsorption from dehydration or rehydration temperatures as a function of the water vapor pressure because the dehydrated compound cannot be regarded as an inert adsorbent in the process of hydration.
The enthalpy of dehydration of the compounds can be calculated as the sum of some quantities including the electrical energy required to displace the anions and the enthalpy of partial hydration of the anions concerned in the gaseous state.
INTRODUCTION
One type of calcium aluminate hydrate
consists of main layers CatAl(OH),j+ alter- nated by interlayers in which anions are sur- rounded by water molecules (14). The type of anions to be incorporated can vary widely;
in the present investigation only univalent
anions have been considered.
process of dehydration (hydration) of the
compounds mentioned.
EXPERIMENTAL
by measuring dehydration and rehydration
temperatures at different water vapor pres-
sures in order to obtain more insight into the
Lavanant (5) examined the dehydration
and rehydration of some of these calcium
aluminate hydrates as a function of tempera- ture and water vapor pressure and, in spite
of the occurring hysteresis, he used the
Clausius-Clapeyron equation to establish the
enthalpy of dehydration. It is questionable
whether this is correct, because the dehydra-
tion is irreversible in the thermodynamic
sense. One of the objects of our investigation (6-8) on the calcium aluminate hydrates was the comparison of the enthalpies of dehydra-
tion, determined both calorimetrically and
The calcium aluminate hydrates were pre- pared and analyzed as described previously
2[CazAl (OH)G+X-* yHzO] (s) +
(6, 7). The enthalpies of dehydration were
determined by measuring the enthalpies of
2[CazAl(OH)G+X-](s) + 2yHzO(l),
Cl1
370
solution of the hydrated and the dehydrated compounds in 1 N HCl or 1 N HClOh (7) in an LKB 8700-l precision calorimeter. If small amounts (about 0.2 mmoles) of the sample are dissolved in about 84 ml hydrochloric acid (1 N), the heat of dilution caused by the formation of water on dissolution of the cal- cium aluminate hydrate is negligible. More- over, if we dissolve about the same amount
of the dehydrated and the hydrated com-
pound, the difference of the heats of solution
per mole calcium aluminate hydrate gives
directly the enthalpy change of the reaction:
Jouvnal of Colloid and Interface Science, Vol. 56. No. 2. August 1976 Copyright @ 1976 by Academic Press, Inc. All rights of reproduction in any form reserved.
DEHYDRATION CALCIUM ALUMINATES 311
which is by definition the enthalpy of de- hydration, AHdeh.
Dehydration and rehydration of the com-
pounds were followed thermogravimetrically by means of a micro balance, CL Electronics type Mark II, equipped with a so-called uni- versal attachment in order to maintain a con-
trolled atmosphere above the sample. A
constant water vapor pressure was main-
tained by passing an air flow, after circulation
through potassium hydroxide solutions of
known concentration, over the sample. The
relative humidity of the air at 20°C was mea-
sured by means of an hygrometer (Hygro-
dynamics Inc.) calibrated with relative hu-
midities above saturated solutions of var-
ious salt hydrates (9). A thermogram of
the dehydration of 2[CazAl(OH)B+X-.2H20] (X- = Br-, ClOh-) was composed by raising the temperature in steps of 5 or 1°C waiting at each temperature 5 to 24 hr until no or no further weight loss was observed. The “de-
hydration temperature”, tD, was defined as
the temperature at which the dehydration,
as measured by the procedure described, has advanced for 50%. A similar definition was used for the rehydration temperature, 1~.
RESULTS
The enthalpies of dehydration of the cal-
cium aluminate hydrates found calorimetrically are given in Table I, column a. The dehydra-
tion and rehydration temperatures found
thermogravimetrically at different relative
humidities (r) of the atmosphere above the
sample are given in Table II for the compound with X- = Br-. If we appIy the Clausius- Clapeyron equation to this data, the apparent
enthalpy of dehydration “AH” follows from
the linear relation between log (r) and l/Tn
(TD = dehydration temperature in Kelvin) ;
“AH” = 71 f 5 kJ/mole HzO. AH, the real
enthalpy of dehydration corresponding to the
TABLE I
The Enthalpies of Dehydration of the Calcium MU- minate Hydrates 2[CazAl(OH)s+X-.yHpO] (y = 2 if X- = C104, NOa-, Cl-, Br-, J-, and ClOs-; 2 <:y < 4 if X- = Br03, JO,-) ; the Enthalpies of Hydration of the Individual Anions in Aqueous Solution and Their Lyotropic Number X- JO,- BrO,- Cl- Br- ClOs- J- NOa- CIO, *IL (kJ/mole hydrate) 159.8 95.8 83.3 77.8 64.9 64.0 54.0 44.8 b -AHhydr LyotCropic (X-3 &bs number (kJ/sr ion) W) 408.3 6.25 331.8 9.55 320.9 10.00 287.4 11.3 305.0 10.65 258.6 12.5 280.7 11.6 275.7 11.8 as follows : i T
n.AH = AHdeh + AC9 ’ dT
298
+ n. AHL+G (J&O) [Z-J (n = 4 in the case X- = Br-, at room tem-
perature and 37% relative humidity), where
ACp is the difference between the heat ca- pacities of n mole HzO(g) together with one
mole of the dehydrated calcium aluminate
hydrate and the heat capacity of the hydrated compound ; AHL+G(HzO) is the enthalpy of evaporation of one mole Hz0 at temperature to. If we assume ACp = 0, it follows from Eq. [2] with AHL+G(HzO) = 42.05 kJ/mole and the AHd,h-value of the calcium aluminate
hydrate concerned as given in Table I,
TABLE II
The Dehydration- and Rehydration Temperatures of the Reactions: 2[Ca2Al(OH)6+Br-.2Hz0] (s) Z 2[CazAl(OH)s+Br-l(s) + 4HzO(g) Relative humidity (%) at 20°C tD (“C) tP. (“0 19 69.2 60.0 30 75.0 67.2 5.5 83.2 74.3
process occurring in the thermobalance is _,
connected with AHdeb, as defined by Eq. [l],&# 7o 88.0 79.0
AH = 61 f 2 kJ/mole H20. Similarly, for
the hydrate containing C104-, in this way
AH = 53 f 2 kJ/mole Hz0 is found. The
“AH”-values are respectively 71 f 5 and
6.5 f 5 kJ/mole HzO. The difference between
the values obtained calorimetrically and
from rehydration or dehydration temperatures distinctly surpasses the experimental error.
DISCUSSION
(a) Comparison of the Enthalpies of Dehydration Determined Calorimetrically and
by Thermogravimetry
The application of the Clausius-Clapeyron
equation on dehydration and rehydration
temperatures at different water vapor pres- sures is suspect because of lack of equilibrium (cf. the occurring hysteresis). However, at any temperature the relationship ln p&h < In pequil < In Pr.3hydr will hold ; thus it is
probable that :
dehydration or rehydration (6). Thus the
fact that the isosteric heat of dehydration as
calculated from thermogravimetric results is
a differential heat, whereas calorimetry yields the integral heat, cannot be accepted as ex- planation for the observed discrepancy. Rather it should be realized that the isosteric heat of
dehydration as found from d In p/a(l/T)-
slopes only coincides with the real dehydration enthalpy (AH) as defined in Eq. [2], if the
dehydrated compound may be regarded as
an inert adsorbent in the process of hydration. Formally, the calcium aluminate hydrate may be treated as a solid solution of variable water
content (10). The enthalpy change of the
reaction :
(lln)A . n&O + (l/n)A + &O(g), C31 where
A.nH20 = 2CCazA1(OH)gtX-.2HzO]
A = 2[CazAl(OH)6+X-1, is equal to:
(if this relationship would not hold, then a
rehydration equilibrium establishment would
be particularly good at low temperatures,
which is very improbable). Since
h(HzO, g> + (llnh
+ HWbO, B) - (lln)W$ B) L41
(A. %HzO = B)
h(HzO, g) = the enthalpy of one mole Hz0 in the gas phase
this would be an argument for identifying both with
H(H20, B) = the partial molar enthalpy of one mole Hz0 in the crystal hydrate con- sidered as solid solution
~3 In p ( a(w7 > ccl’
The difference between the values for AH&h
found calorimetrically and thermogravi-
metrically cannot be ascribed to differences in degree of crystal perfection. It is true the former includes all crystals present in the sample (even very small or partially disordered ones), whereas the latter may reasonably be
thought to be restricted mainly to well-
ordered crystals, but d In p/e (l/T)-values are
only slightly dependent on the degree of
H(A, B) = the partial molar enthalpy
of one mole of the dehydrated compound in the crystal hydrate considered as solid solution. By measuring the dehydration temperature as function of the water vapor pressure we determine the difference between the enthalpy of water in the gaseous state and the partial molar enthalpy of water in the crystal hydrate :
[a In p/X%
From calorimetry, on the other hand, we find (after taking into account the enthalpy of = (h(Hz0, g) - H (HzO, B))/RT2.
vaporization of the water) per mole of water
AH = - (l/n)h(B) +. (l/n)h(A) +
h(Hz0,
g).Treating again the calcium aluminate hy-
drate as a solid solution of variable water content we can replace h(B) by H(A, B) + nH (HzO, B). Thus:
AH = - (l/n)H(A, B) - H(Hz0, B)
which will be identical with [a In ~/8(1/T)&
only when H(A, B) = h(A), i.e., when A
may be regarded as an inert adsorbent.
From our data we can conclude that hi is
a more negative quantity than the partial
molar enthalpy H (A, B). This means that
during hydration of 2CCa2Al(OH)6+X-] part
of the enthalpy change is required for changing
the “adsorbent.” The occurring hysteresis can
be explained by comparing the calcium alumi- nate hydrates with the analogous clay minerals like montmorillonite, vermiculite a.o. (11, 12).
(b) Comparison of the Enthalpies of Dehydration
of Calcium Aluminate Hydrates
Containing D<fekrent Anions
The enthaIpies of dehydration (AH& of
the calcium aluminate hydrates, as determined calorimetrically, increase in the sequence X- = Cl04 < NOa- < J- < ClOs-
< Br- < Cl- < BrOz- < JO,. It was expected that there should be a relation between AHdeh of 2[CazAl(OH)G+X-.yHzO]
and the enthalpies of hydration, AHhydr, of
the anions X-. For X- = halide ions, AHbydr
is known from Born-Haber cycli (13); how-
ever, for X- = oxy-anions the calculation concerned cannot be carried out since the enthalpies of formation of the anions in the gaseous state are not known. AHhydr for oxy- anions is often calculated by methods (13-17) involving cationic and anionic radii. However, these radii are in general not uniquely defined (cf. the existence of very different sets of ionic radii such as those of Pauling, Gold-
Schmidt, Waddington, Gourary and Adrian) ;
in addition, the consideration of oxy-anions as spherical anions is not realistic and the dimensions of an anion depend strongly on the electric field (18) exerted by surrounding ions, which makes the use of one ionic radius for an anion in different solids and in the gas phase doubtful.
We therefore estimated AHhydr of the oxy- anions by the method proposed by Morris
(19) and Waddington (17). It is based on
the existence of a linear relation between the enthalpy of hydration and the so-called
“lyotropic number” (20-22) in those cases
where both can be measured (halide ions); for oxy-anions only the lyotropic number can be established. This can be used to estimate AHhydr by assuming that the same linear relation between AHhydr and the lyotropic number is valid for oxy-anions as for halide ions. Since the lyotropic number is connected with the Gibbs free energy of hydration rather than with AHhydr (6), the assumption mentioned is identical with the hypothesis
that at a given enthalpy of hydration, the
entropy of hydration is fixed, and that the
relation between them is the same for oxy- anions and for halide ions.
“Absolute” enthalpies of hydration have
been calculated (Table I, column b), de- fined by:
AHhydr(X-, g)abs = AH+‘(X-, aq)abs
- AH?@-, d Csl
AHfO(X-, aq)abs = AHfO(X-, aq)conv - AHp(H+, aq)abs. [6] Here AHfO(X-, aq)conv is the “conventional” enthalpy of formation of X- in aqueous solu- tion as found in the literature (see, e.g., (23)) ; AHfO(H+, aq)abs = 400 kJ gramion-l (1.5).
In Fig. 1, the enthalpies of dehydration of the calcium aluminate hydrates are shown versus AHhydr (X-, g)abs. Two linear relations with different slopes are found; moreover, the
enthalpies of dehydration of the calcium
aluminate hydrates are only 10-E% of the
enthalpies of hydration of the anions concerned. In order to understand this, the process of
DEHYDRATION CALCIUM ALUMINATES 373
L,li I
I
Ii
250 300 350 400 45(- - bHhydr (X-,g)ahs. WJ/gr. ion)
FIG. 1. The enthalpies of dehydration of the calcium aluminate hydrates 2[CaaAl(OH)6+X-*yH20]
versns the “absolute” enthalpies of hydration of the individual anions X- in aqueous solution.
hydration of a calcium aluminate hydrate
2[Ca2Al(OH)s+X-] is divided into two hy-
pothetical steps :
(a) the CazAl(OH)e+-layers and X- ions are displaced to their final positions (in the
hydrated compound) without water being
taken up,
(b) water is introduced at constant inter- layer distance.
The enthalpy required by step (a) com- prises an electrostatic term (AB) and the enthalpy required for overcoming non-Cou-
lombic interaction between X- and the
CatAl(OH)G+ layers (AHI). AE can be esti-
TABLE III
The Difference in the Electrostatic Interaction En- ergy of the Anions X- with the CazAl(OH)G+-Layers When they are Displacedover a Distance AC Away from the Main Layers
X- AC (6) (nm) AE (6 = 6) (kJ/mole hydrate) cl- 0.096 97.1 Br- 0.102 102.9 J- 0.120 120.9 NOa 0.132 133.9 c103- 0.112 113.4 BrOa- 0.144 145.6 JO3 0.185 184.9 clod- 0.142 143.5
mated by van Olphen’s method (11): the negative charge of the anions X- is thought to be smeared out in a plane through the middle of the interlayer, the positive charge
of the Ca2Al(OH)e+-layers thought to be
placed in a plane midway of these layers. So the crystal lattice is considered as a number of plate condensers. We obtain
AE = 29r~~Ac. 2NA,‘A/e,
r71
AC = G - c’ = change of the basal basing
on dehydration (cm) ; u = charge density
of the hypothetical surfaces = e/28.63. 1Ol6
esu cmd2; e = 4.803*10-10 esu ; NAV = Avo- gadro number; A = the surface of a unit cell
(3); E = effective dielectric constant (esti-
mated to be 6, based on dielectric constants of comparable solid compounds). The differ- ences of the basal spacings as determined by X-ray diffraction (6), AC, and BE values are given in Table III.
The enthalpy change of process “b” may be compared with the enthalpy of anion X- in aqueous solution, provided the fact is taken into account that water molecules in the crystal lattice have as their nearest neighbors fixed hydroxyl groups (as distinguished from
water molecules in a dilute solution).
We will take into account this fact by two correction quantities, AH11 and AH,,.
DEHYDRATION CALCIUM ALUMINATES 375
AHrr represents the enthalpy change on
account of the interaction of the water mole-
cules with the hydroxyl groups of the
CaTAl (OH)6+-layers on introducing water into
Ca2Al (OH)G+X- previously brought to dis-
tance (step ‘(a”). AHrrr represents the enthalpy change on disrupting water to water bonds in dilute solution.
The enthalpy of hydration of a calcium
aluminate hydrate can then be expressed by the equation :
AHh,d,(complex) = AE + AHI + AHII
+ A&II + =&rir(X-, g)abs. [S]
It is reasonable to assume that the sum of
AH11 + AH111 is constant (independent of the type of anion). So the different slopes of the curves drawn in Fig. 1 are caused by the contribution of AHr. It means that the hydro-
gen bonds between the anions and the
Ca2A1(OH)sf-layers in the dehydrated crystal lattice differ more in the case of the halides than in the case of the oxy-anions. The sum
of AHI + ANrr + AH111 is rather large com-
pared with the enthalpy of hydration of the calcium aluminate hydrate, even after taking into account the electrostatic energy required for the displacement of the anions away from the Ca2Al(OH)6+-layers. However, it makes
perhaps more sense to compare the enthalpies of hydration of the hydrates with the enthalpies of partial hydration of anions X- in the gaseous state.
Recently some data about those enthalpies were published by Arshadi, Kebarle, Payzant et al. (24-27). The enthalpy of hydration of the caIcium aluminate hydrate can then be expressed by the equation
A&,dr(COmplex)
= AE + ‘2. AHo,t(X-, g> + AHI + AHII. [91
In Fig. 2 the separate contributions are shown. It appears that the enthalpy of hydration of the calcium aluminate hydrate is nearly equal to 2AHo,z(X-, g) + AE (taking into account the uncertainty in the quantity AE because of the estimated value for e( =6)). This is true, also in the case X- = NO,-. For the other anions a comparison is not possible since no data about the partial hydration enthalpies are available.
CONCLUSION
During the hydration of the dehydrated cal- cium aluminate hydrates 2[Ca,A1(OH)6+X-] part of the enthalpy change is required to
change the “absorbent.” The enthalpy of
- lyotropic number
FIG. 2. A, @ : -AHdeh (hydrate); X : 2 *AH ilydr (X-, g) abs; q , 0: Z*AHI,~~~ (X-, g) abs -i-AE;
$ : 2AHo.z (X-, g); 8 : 2AHo.a (X-, g) +AE.
hydration of the compounds mentioned can be calculated from the enthalpy of partial hydration of the anion X- in the gaseous state after correcting for the enthalpy required to displace the anion X- over a certain distance away from the main layers.
The enthalpies of dehydration (hydration)
of the compounds cannot be established by
measuring the dehydration or rehydration
temperatures at different water vapor pres- sures because of lack of equilibrium. More- over, the apparent enthalpy of dehydration determined with the Clausius-Clapeyron equa- tion is only the difference of the molar enthalpy of water vapor and the partial molar enthalpy of water intercalated in the calcium aluminate hydrate and does not include the enthalpy
required to change the “adsorbent” during
the process of hydration or rehydration.
ACKNOWLEDGMENT
We thank Mr. C. L. M. Holten, Mr. J. P. M. van Seters, and Mr. F. E. A. M. B. Lemmerling for carrying out the experiments.
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